If audited, you must have some reasonable method to determine the fair market value of the item, that you can show the auditor. Thrift shop value may be the easiest (whatever the thrift shop will charge) but it is not mandatory. In fact, you can claim fair market value for an item even if you know the thrift shop will sell it for less; for example, I donated a Bose radio that sells on eBay for $100, knowing the thrift shop would probably charge $10 or $20 (and some scavenger probably would snap it up and put it on eBay for a profit). I didn't want to deal with the hassle of making an account on eBay, listing it, shipping it, and so on. But I am entitled to the fair market value as a tax deduction, as long as I can prove it. Likewise, I donated a piece of furniture to Goodwill that I could sell on Facebook Marketplace for $50, because I didn't want to deal with the hassle of selling it myself. I can claim the FMV even if Goodwill will charge less.
So it's really up to you, what amount of research you want to do, what kind of proof you want to document and save with your tax papers for 3 years in case of audit. If the easiest thing is to walk through the store, noting the prices they charge for similar items, and that seems like a fair indication of value, then use that method. But you can use other methods if they are more appropriate for particular items.
